The present invention concerns an identification system, and in particular a code reader, for reading one- and/or two-dimensional codes that are arranged at different distances, that has a housing in which a position resolving light receiver and associated focusing optics are arranged.
Known identification systems project an image of a code onto an image plane via a projection optics. A position resolving light receiver is located in the image plane and has multiple light-receiving pixels arranged linearly or in matrix form.
The differing light contrasts of the code are reproduced by the projected image and cause varying photocurrents in the individual light-receiving pixels of the light receiver which can be used to generate signals which identify the content of the code.
Such coding systems have many applications. For example, they can be used to identify and/or control individual objects in transportation systems. The present invention is not limited to processing particular codes and applies to all types of information carriers which can be photoprocessed.
Efficient coding systems are expected to contain increasing amounts of information on coding surfaces that are as small as possible. This requires that the identification system have a high spatial resolution. Particularly high demands are placed on the coding system when position or location of the code can vary over a relatively large distance range. Efficient coding systems must further exhibit a high degree of readability, which requires, amongst others, efficient projection optics. These two requirements, that is, the ability to generate high quality images of objects that can be spaced apart over a wide range, and at the same time provide efficient projection optics, demand that the coding system be equipped with an automatic distance adjuster.
Known distance adjusting systems compensate for different object distances by adjusting what is commonly referred to as the back focal distance, which is the distance measured from the vortex of the last back surface of the last lens of the optics to the focal point (hereinafter usually referred to as the “back focal length”) as a result of changing the distance between the projection optics and the position resolving light receiver in the image plane.
In such situations, it was customary to move the entire receiving optics along the optical axis, while the position resolving light receiver is stationarily mounted in a housing. This is usually attained by moving the projecting optics with a servo motor via a threaded spindle that is engaged by a threaded sleeve. Use of such an arrangement over a long period of time can subject the threaded drive to as many as about 107 to 109 operating cycles over its life. This subjects mechanical bearings and sliding surfaces to extensive wear and tear that can exceed the capability of such components. In addition, such distance adjusting systems operate relatively slowly, so that such systems are not well-suited for applications where a rapid response to changing code distances is required.
U.S. Pat. No. 6,801,260 discloses an arrangement in which a movable mirror is arranged between fixed projecting optics and a fixed light receiver. The mirror is moved either linearly or pivotally to change the distance to the projecting optics. This changes the back focal length so that the images on the position resolving light receiver of objects with different object distances are always sharp and clear.
When the mirror is moved linearly, the point where the optical axis strikes the light receiver changes. This is disadvantageous because for each distance setting the image of a different area or portion of the object strikes the center of the light receiver.
The problem of different object areas or portions at the center of the light receiver can be avoided by moving the adjustable mirror not only linearly but instead pivoting it while at the same time adjusting its distance from the projecting optics. In this manner, the optical axis, and therewith the same object area or portion, is always projected onto the center of the light receiver so that for different distance settings the same object portion is reproduced at the center of the light receiver.
A drawback of the system disclosed in U.S. Pat. No. 6,801,260 is that when the object distance varies, the quality of the projected image suffers. The reason for this is that when the mirror is pivoted, the optical axis of the receiving light beam no longer optimally strikes the position resolving light receiver. The requirement, often referred to as the “Scheinpflug-rule”, concerning the proper orientation and alignment of the object, image and image plane over the range of possible distance settings, is no longer fulfilled. Consequently, projecting errors and distortions result, which lowers the quality of the image.
Such image projection errors reduce the position resolution of the coding system and are therefore undesirable.